High frequency cable, high frequency coil and method for manufacturing high frequency cable

ABSTRACT

A high frequency cable includes: a central conductor made from aluminum or an aluminum alloy; a covering layer made from copper covering the central conductor, and having a fiber-like structure in a longitudinal direction; and an intermetallic compound layer formed between the central conductor and the covering layer and having greater volume resistivity than the covering layer, wherein a cross-sectional area of the covering layer is 15% or less of an entire cross-sectional area including the central conductor, the intermetallic compound layer and the covering layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

The application is a Continuation of PCT Application No.PCT/JP2011/056984, filed on Mar. 23, 2011, and claims the benefit ofpriority from the prior Japanese Patent Application No. 2010-066793,filed on Mar. 23, 2010, the entire contents of which are incorporatedherein by reference.

BACKGROUND

The present invention relates to a high frequency cable and a highfrequency coil, and particularly to a high frequency cable used as awinding wire, a litz wire, a cable and the like for various highfrequency devices, and to a high frequency coil.

In winding wires and feed cables of devices (such as a transformer, amotor, a reactor, induction heating equipment, a magnetic head assembly)in which a high frequency current flows, eddy-current losses are causedin conductors due to magnetic fields generated by the high frequencycurrents, and, as a result, alternating-current resistance is increased(skin effect and proximity effect are enhanced), thus causing heatgeneration and an increase in power consumption. As a general measure tosuppress enhancement of skin effect and proximity effect, a reduction ofwire diameter and use of litz wire in which each wire has insulatingcoating are adapted (for example, refer to Japanese Unexamined PatentApplication Publication No 2009-129550 (hereinafter referred to as“Patent Literature 1”), Japanese Unexamined Patent ApplicationPublication No. Showa 62-76216 (hereinafter referred to as “PatentLiterature 2”), Japanese Unexamined Patent Application Publication No.2005-108654 (hereinafter referred to as “Patent Literature 3”),International Publication No. WO2006/046358 (hereinafter referred to as“Patent Literature 4”) and Japanese Unexamined Patent ApplicationPublication No. 2002-150633 (hereinafter referred to as “PatentLiterature 5”)).

However, with the means of the prior art, it is difficult to remove theinsulating thin layer during a soldering process for the conductorconnection, and there is a limit to wire diameter reduction because thenumber of wires increases. In addition, no effective measures have beenfound to suppress proximity effect for wires having diameters with whichproximity effect is overwhelmingly predominant compared to skin effect,and it is commonly known that characteristics obtained by the diameterreduction measure have limitation. Although examples of countermeasuresare presented in Patent Literatures 1 to 5, all of those countermeasuresare ideas only and short on specifics, and cannot be regarded aseffective countermeasures.

Moreover, in Patent Literature 2, a plurality of composite conductorsmade from a central conductor and an outer conductor are twistedtogether and recrystallized through a thermal treatment to manufacture ahigh frequency cable. However, with this cable, it is difficult tosufficiently suppress proximity effect, and damage and deformation occureasily during manufacturing processes, so it has been difficult tosufficiently stabilize the characteristics of the cable as a coil.

SUMMARY

In view of the aforementioned problems, an object of the presentinvention is to provide a high frequency cable and a high frequency coilwhich can suppress alternating-current resistance and can suppress heatgeneration and power consumption.

According to an aspect of the present invention, a high frequency cableis provided, which is provided with a central conductor made fromaluminum or an aluminum alloy, a covering layer which is made fromcopper that covers the central conductor and has a fiber-like structurein a longitudinal direction, and an intermetallic compound layer whichis formed between the central conductor and the covering layer and hasgreater volume resistivity than the covering layer, and across-sectional area of the covering layer is 15% or less of an entirecross-sectional area which includes the central conductor, theintermetallic compound layer, and the covering layer.

According to another aspect of the present invention, a high frequencycoil in which a high frequency cable is used is provided, where the highfrequency cable is provided with a central conductor made from aluminumor an aluminum alloy, a covering layer which is made from copper thatcovers the central conductor and has a fiber-like structure in alongitudinal direction, and an intermetallic compound layer which isformed between the central conductor and the covering layer and hasgreater volume resistivity than the covering layer, and across-sectional area of the covering layer is 15% or less of an entirecross-sectional area which includes the central conductor, theintermetallic compound layer, and the covering layer.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view showing an example of a high frequencycable according to an embodiment of the present invention.

FIG. 2 is a cross-sectional view showing a copper wire according to acomparative example.

FIG. 3 is a cross-sectional view showing a copper-clad aluminum wireaccording to the comparative example.

FIG. 4 is a schematic view for explaining skin effect according to theembodiment of the present invention.

FIG. 5 is a schematic view for explaining proximity effect according tothe embodiment of the present invention.

FIG. 6 is a graph showing a relationship between skin effect depth andfrequency (theoretical values in a single wire model of a single cable)according to the embodiment of the present invention.

FIG. 7 is a graph showing a relationship between skin effect andfrequency (theoretical values in a single wire model of a single cable)with regard to a copper wire and an aluminum wire according to theembodiment of the present invention.

FIG. 8 is a graph showing a relationship between proximity effect andfrequency (theoretical values in a single wire model of a single cable)with regard to the copper wire and the aluminum wire according to theembodiment of the present invention.

FIG. 9 is a schematic view of magnetic fluxes and leakage fluxes due tocurrents in a magnetic core of a transformer model, in which anassembled conductor according to the embodiment of the present inventionis winded around an iron core.

FIG. 10 is a graph showing properties (theoretical values) of an exampleof a high frequency transformer according to the embodiment of thepresent invention.

FIG. 11 illustrates a segment model of an eddy current and an equivalentcircuit thereof according to the embodiment of the present invention.

FIG. 12 is a table showing structures and properties (actual measuredvales) of a high frequency cable according to the embodiment of thepresent invention and a high frequency cable according to thecomparative example.

FIG. 13( a) is a photograph from an optical microscope showing a crosssection of a worked structure of tough pitch copper (TPC) manufacturedusing an SCR method, and FIG. 13( b) is a photograph from an opticalmicroscope showing a cross section of a worked structure of a copperwire manufactured using a dip forming method.

FIG. 14( a) is a photograph from an optical microscope showing a crosssection of a recrystallized structure of tough pitch copper (TPC)manufactured using the SCR method, and FIG. 14( b) is a photograph froman optical microscope showing a cross section of a recrystallizedstructure of a copper wire manufactured using the dip forming method.

FIG. 15 is a schematic view showing an example of wire drawing diesaccording to the embodiment of the present invention.

FIG. 16 is a schematic view showing classification of shear stressduring wire drawing.

FIG. 17( a) is a schematic view (No. 1) showing an analysis of stressdistribution during wire drawing according to the embodiment of thepresent invention, FIG. 17( b) is a schematic view (No. 2) showing ananalysis of stress distribution during wire drawing according to theembodiment of the present invention, and FIG. 17C is a schematic view(No. 3) showing an analysis of stress distribution during wire drawingaccording to the embodiment of the present invention.

FIG. 18 is a photograph of an observation from a transmission electronmicroscope showing an interface between a covering layer and a centralconductor according to Example 1 of the present invention.

FIG. 19( a) is a graph (No. 1) showing an energy dispersive X-rayspectrometry (EDS) according to Example 1 of the present invention, FIG.19( b) is a graph (No. 2) showing an EDS according to Example 1 of thepresent invention, FIG. 19( c) is a graph (No. 3) showing an EDSaccording to Example 1 of the present invention, and FIG. 19( d) is agraph (No. 4) showing an EDS according to Example 1 of the presentinvention.

FIG. 20( a) is a top view of a reactor according to Example 2 of thepresent invention, FIG. 20( b) is a side view of the reactor accordingto Example 2 of the present invention, and FIG. 20( c) is another sideview of the reactor according to Example 2 of the present invention.

FIG. 21 is a table showing properties (actual measured values) of thereactor according to Example 2 of the present invention.

FIG. 22 is a graph showing properties (actual measured values) of areactor according to Example 3 of the present invention.

FIG. 23 is a table showing properties (actual measured values) of thereactor according to Example 3 of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENT

Next, an embodiment of the present invention will be explained withreference to the drawings. In the following descriptions of thedrawings, same or similar parts are denoted by same or similar referencenumerals. However, it should be noted that the drawings are schematicviews, and relations between thicknesses and plan surface measurements,a thickness ratio of each layer and so on are different from reality.Therefore, specific thicknesses and measurements need to be determinedin consideration of the following explanations. Also, needless to say,some measurement relationships and ratios are different among thedrawings.

Also, the embodiment described below is to show examples of devices andmethods for embodying technical thinking of the present invention, andthe technical thinking of this invention does not limit materials,shapes, structures, arrangements and so on of components to thosedescribed below. Various changes may be made in the technical thinkingof the present invention within the scope of the claims of the patent.

(Structure of High Frequency Cable)

As illustrated in FIG. 1, a high frequency cable according to anembodiment of the present invention includes a central conductor 1 madefrom aluminum (Al) or an aluminum alloy, a covering layer 2 made fromcopper (Cu) which covers the central conductor 1, and an intermetalliccompound layer (an alloy layer) 3 which is formed between the centralconductor 1 and the covering layer 2 such that the composition thereofchanges obliquely from the central conductor 1 through the coveringlayer 2, and has greater volume resistivity than the covering layer 2.

The cross-sectional area of the covering layer 2 is 15% or less of thecross-sectional area of the entire high frequency cable, which includesthe central conductor 1, the intermetallic compound layer 3, and thecovering layer 2. The cross-sectional area of the covering layer 2 ispreferably between approximately 3% and 15% of the entirecross-sectional area, more preferably between approximately 3% and 10%of the same, and even more preferably between approximately 3% and 5% ofthe same. The smaller the ratio of the cross-sectional area of thecovering layer 2 becomes to the entire high frequency cable, the morehigh-frequency resistance can be reduced. It is preferred that thediameter of the entire high frequency cable be approximately between0.05 mm and 0.6 mm.

For the central conductor 1, for example, aluminum for electricalpurposes (EC aluminum) or an aluminum alloy of Al—Mg—Si alloy (JIS6000-series alloy) may be adapted, but an aluminum alloy is preferredrather than EC aluminum because an aluminum alloy has greater volumeresistivity.

The intermetallic compound layer 3 is generated in a wire drawingprocess for a high frequency cable by performing wire drawing of thecentral conductor 1 which is covered by the covering layer 2, using dieseach having a cross-section reduction rate of 20% or higher and combinedinto multiple stages. The thickness of the intermetallic compound layer3 is between approximately 10 nm and 1 μm. The intermetallic compoundlayer 3 contains, for example, Cu₉Al₄, CuAl₂, and so on. The volumeresistivity of the intermetallic compound layer 3 is, for example,between approximately 10 μΩcm and 40 μΩcm, which is greater than thevolume resistivity of the covering layer 2.

Typically, as a winding wire of a transformer, a reactor or the like, acopper wire 100 as illustrated in FIG. 2 which is covered by aninsulating coating of polyurethane, polyester, polyester imide,polyamide imide, polyimide, or the like is used. In a case of a coaxialcable, characteristics of skin effect are taken into considerationbecause of a high frequency current signal, and a copper-clad aluminumwire (hereinafter, referred to as a “CCA wire”) as illustrated in FIG. 3is used in which an aluminum wire 101 has a thin copper layer 102covering outside thereof.

In recent years, devices to which a high frequency current ofapproximately several kHz to several hundreds kHz is applied, such as ahigh frequency transformer, a high speed motor, a reactor, inductionheating equipment, a magnetic head assembly, a non-contact power supplysystem and the like are increasingly used, and, as high frequency cablesused in such devices, winding wires having reduced diameters or Litzwires are generally used for the purpose of reducing alternating-currentlosses. However, it is difficult to remove an insulating thin layerduring a soldering process for the conductor connection, and there is alimit on diameter reduction since the number of wires is increased. Onthe contrary, the high frequency cable according to the embodiment ofthe present invention further enhances a deterrent effect of a wire withreduced diameter to prevent an increase of alternating-currentresistance without using litz wire.

As illustrated in FIG. 4, eddy currents flow within a conductor due tointernal magnetic fluxes, which increases alternating-current resistanceas skin effect. Also, as shown in FIG. 5, an eddy current flows withinthe conductor due to external magnetic fluxes, which increasesalternating-current resistance as proximity effect.

FIG. 6 shows a relationship between frequency and skin effect depth (orskin depth) in a single wire model of a single cable. The skin effectdepth represents a depth from a surface of a cable, at which a currentdensity is 1/e (approximately 0.37) of the surface. As evident from FIG.6, it is understood that an impact of skin effect is small in a casewhere a wire diameter is 0.5 mm (equivalent of twice the skin effectdepth of about 0.25 mm) when a frequency range is approximately 100 kHzor lower.

FIGS. 7 and 8 respectively show frequency characteristics ofalternating-current resistance under skin effect and proximity effect asa ratio between alternating-current resistance (Rac) and direct-currentresistance (Rdc) (Rac/Rdc), on a single wire model of a single cablehaving a diameter of 0.4 mm. In FIG. 8, an external magnetic field H isset to 37.8 A/mm. In a case of FIG. 7 where skin effect is present, anincrease tendency of Rac/Rdc is smaller than the case of FIG. 8 whereproximity effect is present. On the other hand, in the case of FIG. 8where proximity effect is present, Rac/Rdc increases significantly asfrequency goes higher. This tendency of increase depends on a magneticfield strength. In other words, proximity effect is a predominant causeof alternating-current losses in a thin winding wire due to highfrequency currents. Further, it is found from these theoreticalcalculation results that characteristics of proximity effect are smallerin an aluminum wire than in a copper wire. As a measure againstproximity effect, it was proved that increasing volume resistivity of aconductor is an effective method, in addition to reducing a wirediameter of a conductor to the extent possible. However, there is alimit to an increase of volume resistivity, so it is preferred that aconductor material be selected from generally-used materials. Betweencopper and aluminum which are general-purpose conductor materials,aluminum whose conductivity is 61% of copper has better characteristicsin reducing proximity effect. Meanwhile, in a case of aluminum, asurface thereof is covered with an oxide film, and it is extremelydifficult to remove the oxide film especially from a thin wire which isused as a measure against proximity effect. Therefore, we focused on aCCA wire, in which an aluminum wire is covered by thin copper on theouter side thereof.

On the other hand, in a case of a CCA wire, since the volume resistivityof copper is smaller than aluminum, eddy currents generated by anexternal magnetic field are gathered on the copper side and easilycarried in a longitudinal direction of the wire, which means theproperties of aluminum, which has smaller proximity effect than copper,are lost even if aluminum is applied as a central conductor.

A high frequency transformer model is shown in FIG. 9 as an actualexample of high frequency power devices. The high frequency transformermodel includes a magnetic core 10 and first winding wires 11 and secondwinding wires 12 which are winded around the magnetic core 10. Inaddition to magnetic fluxes due to currents flowing in the neighboringfirst and second winding wires 11 and 12, leakage magnetic fluxes fromthe magnetic core 10 also flow in the first and second winding wires 11and 12, so eddy current losses occur due to such external magneticfluxes. Therefore, in the high frequency transformer model, an increasein alternating-current resistance is greater than the single wire modelof a single cable.

FIG. 10 shows theoretical calculation values of frequencycharacteristics of alternating-current resistance as Rac/Rdc with regardto the high frequency transformer model illustrated in FIG. 9. It isalso evident in the case of this actual model that alternating-currentresistance is significantly reduced in an aluminum wire compared to acopper wire. The above-mentioned superiority of an aluminum wireattributes to the fact that aluminum has greater volume resistivity thancopper. On the contrary, aluminum has a difficulty in soldering. Hence,a CCA wire may be considered appropriate as it can cover the shortcomingof the aluminum in practice, but, since a copper layer is provided onthe outer side, eddy currents flow in the copper layer, which ends updeteriorating properties of the aluminum wire.

On the other hand, as illustrated in FIG. 1, according to the highfrequency cable of the embodiment of the present invention, eddycurrents flowing from the central conductor 1 toward the covering layer2 can be suppressed by the intermetallic compound layer 3 which hashigher volume resistivity than the covering layer 2 as illustrated inFIG. 1, thus inhibiting skin effect and proximity effect. Also, sincethe intermetallic compound layer 3 is generated in the interface betweenthe central conductor 1 and the covering layer 2, a thickness of thecovering layer 2 in the wire diameter is equivalently reduced, thusreducing proximity effect. Therefore, alternating-current resistance canbe suppressed without using a twisted wire (litz wire), and heatgeneration and power consumption can be inhibited.

Next, the high frequency cable according to the embodiment of thepresent invention will be explained in contract to a high frequencycable described as a comparative example which is recrystallized througha thermal treatment at a recrystallizing temperature or higher. Sincethe high frequency cable according to the embodiment of the presentinvention is generated by wire drawing of the central conductor 1covered by the covering layer 2 using dies combined in multiple steps,the central conductor 1 and the covering layer 2 become workedstructures and have fiber-like structures in a longitudinal direction asschematically depicted in FIG. 12. Here, a worked structure means acold-worked structure. Cold working means processing conducted at arecrystallizing temperature or lower. Further, a fiber-like structuremeans a structure in which crystal grains are stretched in a drawingdirection by a wire-drawing process. As examples of such workedstructures, FIG. 13( a) shows a cross section of a worked structure oftough pitch copper (TPC) having a diameter of 0.9 mm which ismanufactured by using a SCR (Southwire continuous rod) method, and FIG.13( b) shows a cross section of a worked structure of an oxygen-freecopper (OFC) having a diameter of 0.9 mm which is manufactured by a dipforming method.

Meanwhile, as schematically illustrated in FIG. 12, the high frequencycable according to the comparative example has a recrystallizedstructure which has been recrystallized by conducting a thermaltreatment at a recrystallizing temperature or higher. Here, therecrystallized structure means a structure in which crystal grainshaving strains caused by cold working are replaced by crystals having nostrain through recrystallization. As examples of recrystallizedstructures, FIG. 14( a) shows a cross section of a recrystallizedstructure of tough pitch copper (TPC) having a diameter of 0.9 mm whichis manufactured by the SCR method, and FIG. 14( b) shows across sectionof a recrystallized structure of oxygen-free copper (OFC) having adiameter of 0.9 mm which is manufactured by the dip farming method.

Also, as shown in FIG. 12, the high frequency cable according to theembodiment of the present invention is able to suppress proximity effectbetter since a volume resistivity value thereof is higher than the highfrequency cable according to the comparative example. Moreover, the highfrequency cable according to the embodiment of the present invention hashigher Vickers hardness than the high frequency cable according to thecomparative example, and thus has higher resistance to damages anddeformation at the time of manufacturing, which results in mare stableproperties thereof as a coil.

(Method for Manufacturing a High Frequency Cable)

Next, a method for manufacturing the high frequency cable according tothe embodiment of the present invention will be explained. Themanufacturing method described below is only an example, and amanufacturing method is not particularly limited thereto. The highfrequency cable according to the embodiment of the present invention maybe manufactured in various manufacturing methods.

(a) The central conductor 1 is prepared, which is made from aluminum oran aluminum alloy and has a diameter between approximately 9.5 mm and12.0 mm. The surface of the central conductor 1 is covered with thecovering layer 2 by conducting TIG welding or plasma welding of a coppertape having a thickness of between approximately 0.1 mm and 0.4 mm whilelongitudinally applying the copper tape to the surface of the centralconductor 1. Next, the central conductor 1 covered by the covering layer2 is formed to have a diameter of between 9.3 mm and 12.3 mm by skinpass, thus fabricating a base material constituted by the centralconductor 1 covered by the covering layer 2.

(b) Next, the base material is drawn by being passed through wiredrawing dies at multiple stages. As illustrated in FIG. 15, the wiredrawing dies 20 include an entrance section 21, an approach section 22,a reduction section 23, a bearing section 24, and a back relief section25. The base material 4 is worked at the reduction section 23 to have adiameter d2 which is smaller than a diameter d1 before wire drawing. Inthe embodiment of the present invention, in each of the wire drawingdies, a reduction angle α shown in FIG. 15 is approximately 8 degrees(the entire angle of 2α=16 degrees), and a cross-section reduction rateis approximately 20% or higher per pass (wire drawing die), preferablybetween approximately 20% and 29%. By setting the cross-sectionreduction rate of the wire drawing dies to around 20% or higher, orpreferably between about 20% and 29%, large shear stresses can begenerated continually in the same direction. Due to this shearing heatgeneration, the intermetallic compound layer 3 made from the materialsof the central conductor 1 and the covering layer 2 is formed in theinterface between the central conductor 1 and the covering layer 2.Because the base material passes through the wire drawing dies atmultiple stages, the final diameter of the high frequency cable becomesapproximately 0.6 mm or less.

With the manufacturing method of the high frequency cable according tothe embodiment of the present invention, the intermetallic compoundlayer 3 is formed between the central conductor 1 and the covering layer2 without performing a thermal treatment after wire drawing as thecross-section reduction rates of the dies combined in multiple stages inthe wire drawing process are set to 20% or higher, thus making itpossible to manufacture the high frequency cable illustrated in FIG. 1.

FIGS. 16 and 17( a) to 17(c) show finite element method (FEM) analysisof stress distributions in longitudinal sections when wire drawing isconducted. Following the shear stress classification shown in FIG. 16,FIGS. 17( a) to 17(c) show stress distributions in longitudinal sectionsduring wire drawing when the cross-section reduction rates of the wiredrawing dies are 5%, 10%, and 20%. From FIGS. 17( a) to 17(c), it isproved that a large shear stress is generated when the cross-sectionreduction rate of the wire drawing dies is 20%, compared to the caseswhere the cross-section reduction rates of the wire drawing dies are 5%and 10%. In the embodiment of the present invention, wire drawing isconducted gradually by using a plurality of wire drawing dies having across-section reduction rate of 20% or higher so as to produce greatershear heating continuously and periodically. Therefore, theintermetallic compound layer 3 can be generated in an excellent bondingstate so that the structure thereof changes obliquely between thecentral conductor 1 and the covering layer 2.

Example 1

As Example 1, an intermetallic compound layer 3 was formed between acentral conductor 1 and a covering layer 2 as illustrated in FIG. 1 byusing a plurality of wire drawing dies each having a cross-sectionreduction rate of 20% or higher, and a high frequency cable wasfabricated in which the cross-sectional area of the covering layer 2 was5% of the cross-sectional area of the entire high frequency cable(hereinafter, referred to as a “5% CCA wire”). First, a base materialwas fabricated by welding a 0.15 mm-thick copper tape by TIG weldingonto the central conductor 1 made from aluminum with a diameter of 9.5mm, while applying the copper tape longitudinally thereto, and the basematerial was then formed to have a diameter of 9.25 mm by skin pass.This base material was then passed through wire drawing dies at multiplestages (26 passes) to reduce the diameter from 9.25 mm to 0.4 mm. Thereduction angle α of each of the wire drawing dies was set to 8 degreesflat (the entire angle of 2α=16 degrees), the crass-section reductionrates from the first pass through the third pass were set to between 29%and 24%, the cross-section reduction rates from the fourth pass throughthe tenth pass were set to between 23% and 21%, and the cross-sectionreduction rates from the eleventh pass through the twenty-sixth passwere set to between 21% and 20%.

The copper/aluminum interface of the 5% CCA wire according to Example 1of the present invention was observed using a transmission electronmicroscope (TEM). From the TEM observation, it was confirmed that anintermetallic compound having a thickness of 10 nm or larger wasgenerated in a good bonding condition when the diameter was 1.6 mm afterthe fourteenth pass. Similarly, an intermetallic compound having athickness of 10 nm or larger was confirmed when the diameter was 0.4 mmafter the twenty-sixth pass.

FIG. 18 shows a TEM photograph of the 5% CCA wire. In FIG. 18, the darkarea represents copper, the white area represents aluminum, and the grayarea represents the intermetallic compound layer. It is evident from theFIG. 18 that the intermetallic compound layer 3 is generated in anexcellent bonding condition so that the composition thereof is obliquelyshifted from the central conductor 1 through the covering layer 2. FIGS.19( a) to 19(d) respectively show results of point analysis from energydispersive X-ray spectrometry (EDS) concerning point Pi in the centralconductor 1, point P2 in the intermetallic compound layer 3 on the sideof the central conductor 1, point P3 in the covering layer 2, and P4 inthe intermetallic compound layer 3 on the side of the covering layer 2shown in FIG. 18. As shown in FIG. 19( b), it was confirmed thataluminum atoms were rich in the intermetallic compound layer 3 on theside of the central conductor 1, and, as shown in FIG. 19( d), it wasconfirmed that copper atoms were rich in the intermetallic compoundlayer 3 on the side of the covering layer 2. From FIGS. 19( a) to 19(d),it is evident that metallic materials which constitute the intermetalliccompound layer 3 are distributed obliquely from the central conductor 1through the covering layer 2. Also, Cu₉Al₄ and CuAl₂ are maincompositions of the intermetallic compound layer 3, and volumeresistivity of thin and flat-shaped Cu₉Al₄ and CuAl₂ is approximately 10μΩcm or higher. Since the volume resistivity of copper is 1.724 μΩcm,the volume resistivity of the intermetallic compound layer is at least 5times larger than copper, which is thought to be a sufficient value.

Example 2

As shown in FIGS. 20( a) through 20(c), as an Example 2 of the presentinvention, winding wire is fabricated in which the 5% CCA wire drawninto 0.4 mm is covered with polyurethane (hereinafter, referred to as a“5% CCA winding wire”), and a reactor was fabricated which is providedwith a magnetic core 32 and the 5% CCA winding wire 31 arranged aroundthe magnetic core 32. The number of 5% CCA winding wires 31 used was 14,and the number of winding turns was 80. Also, as comparative examples,reactors using aluminum wires and copper wires were fabricated,respectively. Direct-current resistance and alternating-currentresistance were measured using each of those reactors fabricated.

FIG. 21 shows properties of the 5% CCA winding wire according to Example2 of the present invention in comparison with the aluminum winding wireand copper winding wire. Comparing the 5% CCA winding wire to the copperwinding wire in a reactor in which inductances are adjusted to besubstantially identical, it is understood that alternating-currentresistance is decreased by almost half even if direct current resistanceis 1.57 times higher.

Example 3

In addition to a reactor in which 5% CCA winding wires were usedsimilarly to Example 2 of the present invention, reactors werefabricated as Example 3 of the present invention under the sameconditions as the reactor in which the 5% CCA winding wires were used,by respectively using a winding wire of a high frequency cableillustrated in FIG. 1 in which a cross-sectional area of a coveringlayer 2 is 15% of the cross-sectional area of the entire cable(hereinafter, referred to as a “15% CCA winding wire”), a winding wireof a high frequency cable in which a cross-sectional area of a coveringlayer 2 is 10% of the cross-sectional area of the entire cable(hereinafter, referred to as a “10% CCA winding wire”) , and a windingwire of a high frequency cable in which an aluminum alloy (JIS 6063alloy) was used as a central conductor 1 and a cross-sectional area of acovering layer 2 is 5% of the cross-sectional area of the entire cable(hereinafter, referred to as an “alloyed aluminum 5% CCA winding wire”).

FIGS. 22 and 23 show frequency characteristics of alternating-currentresistance with regard to each of the 15% CCA winding wire, the 10% CCAwinding wire, the 5% CCA winding wire, and the alloyed aluminum 5% CCAwinding wire according to Example 3 of the present invention, as well asthe copper winding wire and the aluminum winding wire according to thecomparative example. From FIGS. 22 and 23, it is proved thatalternating-current resistance is greatly reduced in the 15% CCA windingwire, the 10% CCA winding wire, and the 5% CCA winding wire, compared tothe copper winding wire. Further, it is evident that alternating-currentresistance is significantly reduced in the alloyed aluminum 5% CCAwinding wire, compared to the copper winding wire and the aluminumwinding wire.

Yet further, from the characteristic values of the 15% CCA winding wire,the 10% winding wire, the 5% winding wire, it is evident that thesmaller the ratio of cross-sectional area of the covering layer 2 shownin FIG. 1 is, the smaller alternating-current resistance is. This wasfound out to be mitigation of proximity effect because the wire diameterhad an equivalently reduced thickness of the covering layer 2 as theintermetallic compound layer 3 was generated, in addition to an effectof suppressing eddy currents. In theory, proximity effect is known to beproportional to the fourth power of a wire diameter.

Other Embodiments

The present invention has been described based on the foregoingembodiments, but it should not be understood that the description anddrawings that are a part of this disclosure limit the present invention.From this disclosure, various alternative embodiments, examples, andoperation techniques will be obvious to those skilled in the art.

Although a wire (single wire) was described as the high frequency cableaccording to the embodiment of the present invention, this wire may beused as a multi-core wire in which multiple wires are bunched or litzwire in which a plurality of wires are twisted together, and themulti-core wire and litz wire can also suppress alternating-currentresistance even more effectively.

Moreover, the high frequency cable according to the embodiment of thepresent invention may be applied to various devices such as a highfrequency transformer, a motor, a reactor, a choke coil, inductionheating equipment, a magnetic head, a high frequency feed cable, a DCpower supply unit, a switching power supply, an AC adapter, adisplacement detecting sensor and a flaw detecting sensor for an eddycurrent detection method and the like, an IH cooking heater, anon-contact power supply system such as a coil, a feed cable or thelike, a high frequency current generator, or the like.

When the high frequency cable according to the embodiment of the presentinvention is deformed in the case where the high frequency cable is madeinto a coil or used as litz wire, the High frequency cable is deformedwithout performing a thermal treatment in order to maintain the workedstructure thereof (fiber-like structure in the longitudinal direction).Also, a thermal treatment may be carried out at a temperature lower thana recrystallizing temperature in order to raise resistance values of thecentral conductor 1 and the covering layer 2. In case where a thermaltreatment is conducted, the high frequency cable may be deformed duringthe thermal treatment, or may be deformed before the thermal treatment.Further, a thermal treatment may be performed on the high frequencyentirely or locally.

As explained above, it is naturally understood that the presentinvention include various embodiments that are not described herein.Therefore, the technical scope of the present invention is defined onlyby the invention-defining matters according to the reasonable scope ofthe claims of the invention.

INDUSTRIAL APPLICABILITY

The high frequency cable and the high frequency coil of the present,invention may be used in electronic device industries includingmanufacturing of various devices such as a high frequency transformer, amotor, a reactor, a choke coil, induction heating equipment, a magnetichead, a high frequency feed cable, a DC power supply unit, a switchingpower supply, an AC adapter, a detecting sensor and a flaw detectingsensor for an eddy current detection method and the like, an IH cookingheater, a non-contact power supply system which includes a coil, a feedcable or the like, a high frequency current generator, or the like.

What is claimed is:
 1. A high frequency cable, comprising: a central conductor made from aluminum or an aluminum alloy; a covering layer made from copper covering the central conductor, and having a fiber-like structure in a longitudinal direction; and an intermetallic compound layer formed between the central conductor and the covering layer and having greater volume resistivity than the covering layer, wherein a cross-sectional area of the covering layer is 15% or less of an entire cross-sectional area including the central conductor, the intermetallic compound layer and the covering layer, and wherein a thickness of the intermetallic compound layer is between 10 nm and less than 1 μm.
 2. The high frequency cable according to claim 1, wherein the intermetallic compound layer is formed so as to shift such that a composition thereof obliquely shifts from the central conductor through the covering layer.
 3. The high frequency cable according to claim 1, wherein the intermetallic compound layer is formed by wire drawing of the central conductor covered by the covering layer using dies at multiple steps, each of the dies having a cross-section reduction rate of 20% or higher.
 4. The high frequency cable according to claim 1, wherein volume resistivity of the intermetallic compound layer is 10 μΩcm or higher.
 5. A high frequency coil using a high frequency cable, wherein the high frequency cable comprises: a central conductor made from aluminum or an aluminum alloy; a covering layer made from copper covering the central conductor, and having a fiber-like structure in a longitudinal direction; and an intermetallic compound layer formed between the central conductor and the covering layer and having greater volume resistivity than the covering layer, and a cross-sectional area of the covering layer is 15% or less of an entire cross-sectional area which includes the central conductor, the intermetallic compound layer and the covering layer, and wherein a thickness of the intermetallic compound layer is between 10 nm and less than 1 μm. 